1. Field of the Invention
The present invention relates to a flat-shaped Fe-Ni alloy fine powder particles superior in soft magnetic characteristic and having a mean particle size of 0.1 to 30 .mu.m, preferably 0.1 to 20 .mu.m and a mean thickness not greater than 2 .mu.m, preferably not greater than 1 .mu.m.
2. Description of the Related Art
In recent years, magnetic cards pertaining to personal secret data, typically bank cards and credit cards, are finding spreading use. In recent years, there has been an increasing demand for these magnetic cards coated by a film of fine powder particles of high magnetic permeability materials. In general, powders used as the coating material are required to be fine in size and high in magnetic permeability. In addition, particles of such a powder are required to be flat. High flatness of the powder particle is required not only from the view points of ease of application and smoothness of the film but also from the fact that the powder particles, under shearing force exerted by a coater, are laid flat in parallel with the card substrate so as to minimize the demagnetization factor thereby to provide a high magnetic permeability in the longitudinal direction of the card surface.
Such coating powder is generally required to have a mean particle size of 0.1 to 30 .mu.m, a mean thickness not greater than 2 .mu.m and a coercive force of 400 A/m or less, preferably 240 A/m or less, in a randomly laid state neglecting demagnetization. The term "thickness" is used in the specification to mean the thickness as measured through a microscopic observation of a cross-section of a specimen resin in which the powder has been embedded while being oriented toward the flat direction through the application of magnetic field and then fixed.
Fe-Ni alloy powders are expected to meet requirements for high magnetic permeability and flatness because these alloys inherently have high levels of magnetic permeability and high levels of plasticity which facilitate flattening by plastic work. Unfortunately, however, no method has been developed for enabling mass-production of Fe-Ni alloy powder which would meet the above-described dimensional specifications and properties.
Japanese Patent Laid-Open Publication Nos. 63-35701 and 63-35706 disclose methods in which flaky metallic powders of high magnetic permeability, having thicknesses not greater than 2 .mu.m and a thickness-to-diameter ratio not greater than 1/10 are produced by wet ball-mill process. More specifically, in one of these methods, pure iron powder particles which have passed a sieve of 44 .mu.m mesh are pulverized for 96 hours so as to become flaky powder of about 1.0 .mu.m thick capable of passing a sieve of 25 .mu.m mesh at a rate of 98%. In the other method, powder particles of Sendust alloy which have passed a sieve of 44 .mu.m mesh are pulverized for 96 hours so as to become flaky powder of about 1.0 to 1.5 .mu.m thick capable of passing a sieve of 25 .mu.m mesh at a rate of 96%.
While it is true that these methods can provide magnetic powder of mean thickness not greater than 2 .mu.m, these methods are still unsatisfactory in that they require a pulverizing step which takes a very long time, i.e., 96 hours and in that they are not suitable for production of fine powders of 30 to 20 .mu.m or finer at a high yield. Furthermore, powders produced by these methods exhibit high levels of coercive force due to strain incurred during pulverizing. For instance, the above-mentioned Fe powder and the Sendust alloy are reported to exhibit high levels of coercive force, say 43 Oe (3440 A/m) and 9 Oe (720 A/m), respectively.
Japanese Patent Laid-Open Publication No. 62-238305 discloses a method for producing flat-shaped Sendust alloy powder in which a Sendust alloy is atomized by water-atomization method into grains of grain sizes not greater than 100 .mu.m and these grains are pulverized into single crystals having longer-dimension-to-shorter-dimension ratio of 10 or greater by means of a crusher having a high energy density. The flaky powder produced by this method also exhibit an impractically high level of coercive force due to strain incurred during the pulverization. This method, therefore, cannot suitably be used for the production of magnetic cards shielding powder to which the present invention pertains.
Japanese Patent Laid-Open Publication No. 58-59268 discloses a method in which Sendust powder which have been formed from an ingot through repeated pulverizing steps are subjected to an annealing in hydrogen atmosphere for the purpose of relief of the pulverizing strain. This Publication, however, fails to definitely disclose the level of the coercive force and does not show any practical method of annealing for reducing coercive force. The methods shown in this Publication, therefore, cannot be used satisfactorily in the production of magnetic card shielding powder to which the invention pertains.
Furthermore, all the Publications mentioned hereinbefore do not mention saturation magnetostriction constant.
No prior art example has been found as to a method of producing flat fine powder of permalloy which is a kind of Fe-Ni alloy. Under these circumstances, the present inventors have proposed, in Japanese Patent Laid-Open Publication No. 63-123494, wherein Fe-Ni alloy powder of a mean particle size not greater than 10 .mu.m is formed by water-atomization and then subjected to a mechanical pulverizing so as to become flat-shaped fine powder of mean particle size ranging between 0.1 and 10 .mu.m and thickness not greater than 1 .mu.m. In this Publication, the inventors have pointed out that the Fe-Ni alloy is easy to flatten due to large plastic workability but is difficult to pulverize into finer size. Thus, the inventors made it clear that, from the view point of pulverizing efficiency, it is important to reduce the particle size of the initial powder.
The method proposed in Japanese Patent Laid-Open Publication No. 01-294801 appreciably facilitates production of flat fine powder particles of Ni alloy. Reduction of the initial particle size, however, is not considered to be a good policy for mass-production from the view point of atomization. Namely, the water-atomizing method, though most suitable for mass-production and most effective in the reduction of particle size among various atomizing methods, requires that the melt of the alloy has to be atomized at a water pressure of 1000 kgf/cm.sup.2 or higher when the particle size has to be reduced to 10 .mu.m or below. In consequence, a huge investment is required for installation of piping and a high-pressure water pump, as well as laborious and troublesome maintenance work. In addition, since the beam of the melt has to be restricted to several millimeters in diameter or below, the throughput per unit time is extremely small. In addition, it is not easy to obtain powder of particle size of 10 .mu.m or less at a high yield. Thus, the method proposed in Japanese Patent Laid-Open Publication No. 01-294801 has a drawback in that the mass-production cannot be carried out efficiently when the whole process starting with the preparation of the material powder is considered.
Further, the method of Japanese Patent Laid-Open Publication No. 01-294801 comprises the steps of mixing a non-magnetic refractory material powder with the Fe-Ni based alloy powder, annealing the mixture, and then separating the magnetic Fe-Ni based alloy material by means of magnetic selection. In this method, it is possible to prevent Fe-Ni powder from being coagulated (conglomerated), however, it is impossible to completely separate and remove the non-magnetic refractory material powder from that flat-shaped Fe-Ni based alloy powder because both the non-magnetic refractory material powder and the Fe-Ni based alloy powder are adsorbed each other due to the very fine grain size of 10 .mu.m with respect to the grains of the Fe-Ni based alloy powder.
The existence of some non-magnetic refractory material in the final alloy powder causes a decrease in magnetic permeability even if the flat-shaped Fe-Ni based alloy powder itself has a high magnetic permeability, so that it becomes impossible to obtain the desired product (which is to be used for a magnetic shielding material) having sufficient magnetic shielding ability or effect.
The precursor particles to the flat shaped fine powder particles to which the present invention pertains, namely particles having a mean particle size of 0.1 to 30 .mu.m and mean thickness not greater than 2 .mu.m, are extremely fine and have been heavily strained. Therefore, if this powder were annealed under the same condition as that for usual bulk material, the flat shape attained through pulverizing is impaired due to coagulation of the particle, i.e., sintering. Therefore, the annealing has to be conducted at a temperature which is low enough to prevent the coagulation, much lower than the annealing temperature for the usual bulk material which is generally around 1100.degree. C. Consequently, the conventionally annealing but at lowered annealing temperatures cannot produce any remarkable effect in reducing the coercive force, so that the flat-shaped fine powder produced by the conventional method exhibited a large coercive force of 500 A/m or greater even after an annealing.